Carbon microballoons are employed as low density filler for thermal insulating materials, for instance, polyimides.
In order to decompose a formed degradable material such as a phenolic resin microballoon into carbon and recover the carbon structure intact, the heating and cooling steps of the process must be controlled, in an inert atmosphere, as to temperature, temperature gradient, and time.
U.S. Pat. No. 4,229,425 describes a process in which a continuously heated oven is lined with a muffle comprising an elongated quartz ceramic tube which is heated to raise its temperature to or above the carbonizing temperature of preformed degradable microballoon material. The patent indicates that the degradable microballoon material may be polymers of alkyd or phenol resins or polyurethanes.
The muffle tube glows and transfers heat by radiation to a graphite boat which contains the degradable microballoons to be pyrolyzed. The muffle is heated from the outside in an oxygen atmosphere, but the inside of the muffle is flushed with an inert gas such as nitrogen so that the boat containing the degradable microballoons experiences a continuously changing inert atmosphere. Once the muffle is heated, it is said to be necessary to keep it heated continuously, since it will crack if it is allowed to cool. Therefore the microballoons must be fed into the muffle and removed therefrom as quickly as possible so that the muffle is not seriously cooled by opening it at one end.
Dwell time in the muffle of the graphite boat containing the microballoons is dictated by the necessity of heating the microballoons slowly enough so that they do not simply burst as a result of rising internal gas pressure. The walls of the microballoons are microporous, and gases can diffuse therethrough if the temperature of the batch is raised at a slow enough rate. The boat not only limits the rate of heating of the microballoons but also acts as a sacrificial material which is attacked by oxidizing gases and moisture given off as the microballoons are heated.
According to U.S. Pat. No. 4,229,425, the decomposition heating process takes about four hours, after which time period one end of the muffle is opened and the boat is removed and immediately placed in a second, unheated chamber which is also provided with an inert atmosphere, so that the microballoons and the boat are not oxidized before they can cool below the auto-ignition temperature of carbon microballoons in air. The second chamber also reduces the rate of heat loss through radiation so that the carbonized material cools at a rate which will prevent cracking or deterioration thereof.
The carbon microballoons which result from decomposition are then recovered from the cooled boat, are screened in order to break up any agglomeration of the particles, and are then immediately packaged in airtight containers to keep them out of contact with moisture in the atmosphere. These microballoons are filled with nitrogen at the time when they are removed from the cooling chamber as a result of the processing steps described.
U.S. Pat. No. 4,229,425 indicates that efforts to speed up the process by shortening the heating period of the microballoons has resulted in a lower yield of intact carbon microballoons due to rupturing of the spherical form, cracking, oxidizing, etc. The patent claims a batch process for manufacturing carbon microballoons which includes the steps:                (a) heating a first chamber to a temperature which is above the carbonizing temperature of the porous-walled heat carbonizable microballoon precursor material and within the range 2000–3000° F. (1093–1649° C.) and continuously maintaining the temperature at that level;        (b) enclosing a batch of microballoons within a graphite boat which surrounds the microballoons except for small openings sufficient to pass gases through the boat;        (c) containing and heating the boat and batch in the heated first chamber for about 4 hours while flushing the chamber with an inert gas and until the batch is carbonized to form microballoons; and        (d) removing the heated batch from the first chamber and immediately confining it in a second unheated chamber, and flushing the second chamber with an inert gas until the batch cools below its self-ignition temperature in air.        
As compared to the process described in U.S. Pat. No. 4,229,425, the present process—which comprises an intermediate cure routine—provides carbon microballoons with less breakage and ultimately enables the manufacture of a better foam product.